Synchronization of the TACS/AMPS mobile phone system is based on an eleven bit word synchronization pattern observed at the receiver in the received frame. The mobile telephone determines that the receiver is in synchronization if the eleven bit word synchronization pattern is identical to a reference word.
There is one checking operation for the control channel (FOCC) and one control operation for the speech channel (FVC).
If the two last word synchronizations in the control channel are lost (SFLAG=0 for the time of two frames), and the decoding results for the data shows the data content to be erroneous, a "hunt mode" is initiated. In the hunt mode, the receiver's frame counter is reset to zero and a search is initiated for a new word synchronization pattern to enable receiving data again.
Generally, there are two possible reasons for the lost synchronization words. One reason is that occasional bit errors appear in the word synchronization pattern and the data field, but the phase of the received bits (the phase of the frame counter) may be correct. In such a situation, prior art devices unnecessarily initiate the hunt mode. This may lead to losing the subsequent frame if the new word synchronization pattern is not captured immediately.
Second, the phase may be wrong because the phase lock has slipped, normally 1 . . . n bits leading or trailing the proper phase. In this case, the hunt mode should be set to correct the phase error. However, if the phase error could be detected within the first frame, the hunt mode could be set without delay, thus correcting the error with a minimum of lost time and data.
In the speech channel (FVC), if the last word synchronization has been lost (SFLAG=0), the hunt mode is set.
In the speech channel, the decoding results detect an error in the same way as the control channel. The drawback may be that the hunt mode is set unnecessarily.
As already mentioned, setting the hunt mode unnecessarily may cause the loss of the following frame, if the following word synchronization pattern contains even one bit error. In such a case, the pattern cannot be captured. Frames will continue to be lost until a word synchronization pattern free of errors is received.
The situation described above appear in the prior art. The unnecessary setting of the hunt mode is detrimental, particularly for the user's own speech channel (FVC), since loss of repetitions decreases performance of the phone.
Unnecessary hunt mode also sets the phase lock of the data receiver to a broad band operation, which causes excessive jitter in the receiver clock, and can cause slipping of the phase lock and loss of correct phase, thus causing bit errors.